Problem: Solve for $x$ and $y$ using substitution. ${-4x-2y = -8}$ ${y = 3x+9}$
Answer: Since $y$ has already been solved for, substitute $3x+9$ for $y$ in the first equation. ${-4x - 2}{(3x+9)}{= -8}$ Simplify and solve for $x$ $-4x-6x - 18 = -8$ $-10x-18 = -8$ $-10x-18{+18} = -8{+18}$ $-10x = 10$ $\dfrac{-10x}{{-10}} = \dfrac{10}{{-10}}$ ${x = -1}$ Now that you know ${x = -1}$ , plug it back into $\thinspace {y = 3x+9}\thinspace$ to find $y$ ${y = 3}{(-1)}{ + 9}$ $y = -3 + 9$ $y = 6$ You can also plug ${x = -1}$ into $\thinspace {-4x-2y = -8}\thinspace$ and get the same answer for $y$ : ${-4}{(-1)}{ - 2y = -8}$ ${y = 6}$